Service Worker

• A service worker is a script (a JavaScript file) that runs in the background, separate from a web page, and provides features that do not need a web page or user interaction. Major features include rich offline experiences (offline-first web application development), periodic background syncs, push notifications, intercepting and handling network requests, and programmatically managing a cache of responses.
• A service worker cannot access the DOM directly. However, it can communicate with the pages it controls by responding to messages through the postMessage interface, and those pages can manipulate the DOM.